This invention relates to a filter unit for filtering a compressed gas contaminated with oil, in particular compressed air, wherein the filter unit contains a coalescence filter for coalescing the contaminant contained in the compressed gas, in particular oil, according to the preamble of the first claim.
Many of today's systems and processes employ compressed gases, especially compressed air, for a variety of purposes, including automation, for inducing a displacement or a movement etc. In typical applications where there is a direct contact with compressed air, such as in the finishing of automobile parts, respiration of human beings, food packaging etc., it is desirable that compressed air is used with a purity which is as high as possible.
Filtering of the gas intake by a compressor, in particular air, is mainly provided to minimize the concentration of the contamination present therein and to reduce wear of the compressor at the position of the suction inlet. Thus, atmospheric air may contain 0.05-0.5 mg/m3 of unburned oil particles.
Filtering of the compressed gas emanating from the compressor, in particular compressed air, mainly envisages the removal of solid particulate matter, water and oil. Oil is mainly present in the form of oil droplets, aerosol and vapor, water is mainly present in the form of water vapor, aerosol, droplets and as a liquid acid concentrate, solid particles may, for example be microorganisms, dust and rust particles. Contamination of compressed air may therefore inter alia be attributed to contamination already present in the intake air, but also to evaporation of the oil and wear in the course of compression when using oil-lubricated air compressors. Numerous air compressors namely use oil in the compression stage, for sealing and lubrication purposes and as a coolant. Oil transfer during compression is typically less than 5 mg/m3, due to the efficiency of the compressors in modern integrated air/oil separators.
Modern compressor installations contain in addition to a compressor, one or more filters and dryers for the purification of the compressed air. The ISO8573-1:2010 air quality standard was developed as a standard method for measuring and representing the purity of air at different positions in a compressor. In order to allow oil removal to a sufficiently low concentration, compressed air is typically subjected to a filtration in several successive steps. For the removal of oil aerosol, and oil vapor from compressed air, use is usually made of a group of two or more consecutive coalescence filters, in particular a pre-filter for removing the bulk of the oil aerosol, followed by a high efficiency coalescence filter which removes the residual oil aerosol up to the envisaged level. In addition to oil aerosol, coalescence filters may also remove very small solid particles, for example particles up to 0.01 micron. Each coalescence filter is positioned in its own housing. To ensure optimum purification, the coalescence filter is generally preceded by a water separator and an activated carbon filter for removing oil vapor. In the course of time assembly kits have been developed, which enable easy mounting of a series of successive filters.
Omitting the removal or reduction of the concentration of the contamination can bring along problems in the installation that makes use of the compressed air, and cause damage or blockage of valves, cylinders, air motors, devices that make use of air, manufacturing plants, and can cause undesirable product contamination. In addition to these problems with the compressed air system itself, the release of particulates, oil and microorganisms may lead to an unhealthy and unsafe environment, Use of contaminated compressed air often leads to inefficient production processes, product waste, reduced production efficiency and increasing production costs.
U.S. Pat. No. 4,124,360 describes a filter apparatus comprising a pre-filter for coalescing the majority of the oil contained in the compressed air supplied by the compressor, and a coalescence medium for coalescing the residual oil present in the air which originates from the pre-filter. The coalescence filter is arranged concentrically with respect to the pre-filter, with a layer of air between the two filters. The coalesced oil which originates from the pre-filter is intermittently removed. It is believed that by carrying out the coalescence in two steps, in particular a coarse filtration followed by a fine filtering, and by intermittently discharging the coalesced oil, a better purification of the compressed air may be obtained. In order to achieve the desired purity, it is of use that the compressed air is filtered by two successive coalescence filters, wherein the first filter is intended to reduce the amount of oil to about 0.1 mg/m3, while the subsequent second filter is intended to further reduce the amount of present in the compressed air to 0.01 mg/m3.
The known filter device which makes use of two consecutive coalescence filters presents the disadvantage that the coalescence filters occupy a relatively large space.
Consequently, there is a need for a compressed air filter unit for an air compressor, which is more compact than those used up to now, without this going at the expense of the purity of the compressed air.